Loop antenna

"A loop antenna has a continuous conducting path leading from one conductor of a two-wire transmission line to the other conductor. All planar loops are directional antennas with a sharp null, and have a radiation pattern similar to the dipole antenna with E and H fields interchanged."[1]

Problems with Earhart's loop antenna

A loop antenna is not a very reliable device in an aircraft at high frequencies. The reason is, that the metal structure and surface of the aircraft creates reflections of the incoming signal -- at ANY frequency -- which can and will create confusion about the exact direction of arrival of the signal. At low frequencies this can be "compensated" for, mechanically and electrically. This compensation consists of distorting the rate of rotation of the loop in certain parts of the compass circle, so that the APPARENT direction as displayed on the bearing indicator gives a more-or-less true heading even though, for instance, there may be a 10-degree error introduced by the wings or tail assembly.

At high frequencies the compensation problem of a loop becomes much more difficult because the actual physical size of the aircraft, in the case of something like a Lockheed 10E or larger, becomes a function of the wavelength of the signal. A quarter-wavelength at 3105 KHz is about 75 feet; at 6210, about 38 feet; at 7500, about 30 feet (rough estimates; I did not take the time to compute actual lengths by formula). The wingspan, the fuselage length, the length of the Vee antenna, all are nearly "resonant lengths" or some fraction thereof at these frequencies. All will create reflections which introduce errors into a loop... MAJOR errors. Given the state of the art in 1937, it does not seem logical to believe that such problems had been solved or at least minimized to a reasonable point... after all, even the LF radio compass was in its infancy, having made the scene in 1935 or so. (Before this, "radio direction finding" in aircraft consisted of fixed loop antennas, so that keeping the aircraft pointed at the station caused a "null" in the pilot's headphones. If the place you needed to fly to was right beside the station, that was fine; but if you needed to find a field located away from the station, there wasn't any way to fly a heading away from it to reach that point, maintaining a reference to the radio station.)